Methods of visual acuity testing in preverbal children

METHODS OF VISUAL ACUITY
TESTING IN PREVERBAL CHILDREN

Presented by :-
Dr. varsha varshney
Resident,
Dept. of ophthalmology.

DEFINITION:

 Visual acuity, in preverbal infants, is defined as a
motor or sensory response to a threshold stimulus
of known size at known testing distance.

 In preliterate but verbal children, visual acuity is
defined as the smallest target of known size at
known testing distance correctly verbally
identified by a child.

DEVELOPMENT AND
MATURATION OF VISUAL
ACUITY

 In order for a visual system to develop normally, several
components are required.

 To receive visual stimulation the anatomical structures
must be present, the two eyes must be positioned correctly
and have clear media.

 The neurological connections of the visual pathway to the
visual cortex must also be functional.

 Compared with the relatively dark environment within the
uterus, the newborn is bombarded with visual stimuli of
differing light intensity and contours within the first few
months of life. This encourages the development of the
lateral geniculate nucleus and striate cortex.

 Structural development is largely complete by 2-3 yrs of life
but functional changes continues throughout life.

 THERE IS ALSO A QUESTION OF NATURE VERSUS NURTURE.

 Does the development of vision rely on the maturation of the
visual system with growth or does it depend upon
environmental stimuli?

 A study comparing premature babies with babies born at full
term, showed that at a given age, the premature babies did not
have better visual acuity(Brown & Yamamoto, 1986), even
though they had had more months of visual stimulation. This
suggests that the anatomical structures have to reach a certain
growth andmaturity before they can all perform at similar
levels.This favours the nature argument.

 Interestingly, in support of the importance of nurture, it has
been shownin recent research by Smith et al (2007), the
absence of a fovea in primates at 3 weeks did not affect
refractive development and emmetropisation still occurred as
long as visual stimulation was normal.

 Visual acuity improves rapidly during the first year of life and
then matures more gradually to adult levels at approximately 5-6
years of age.

 Although the central cones function by term birth, acuity as
measured by the different techniques does not approach 20/20
(6/6) until from 6 to 30months (depending upon the
examination technique used).

 Reasons for this delay include the incomplete development and
specialization of photoreceptors, maturation of synapses in the
inner retinal layers, and myelination of the upper visual
pathways.

 Foveal cones do not attain adult appearance until 4months after
term birth, and visual pathway myelination continues until 2
years of age

DEVELOPMENT AND MATURATION OF
VISUAL ACUITY :

VISUAL MILESTONES :
 Very soon after birth - Can fix and follow a light source, face or large, colorful
toy.

 1 months - Fixation is central, steady and maintained, can follow a slow target,
and converge, preference of looking at face.

 3 months - binocular vision and eye cordination, eyes follow a moving light or
face, responsive smile.

 6 months - Reaches out accurately for toys.

 9 months – look for hidden toys.

 2 years - Picture matching

 3 years - Letter matching of single letters (e.g., Sheridan Gardiner)

 5 years - Snellen chart by matching or naming

Other functions which correlate with vision

VISUAL ACUITY OF INFANT EYES
Test 2Months 4Months 6Months 1Year Attainment
(months)
Opticokineti 20/400 20/400 20/200 20/80 24–30
c nystagmus
test
Forced 20/400 20/200 20/200 20/50 18–24
choice
preferential
looking test
Visual 20/200 20/80 20/60– 20/40– 6–12
evoked 20/20 20/20
response
test

Why to record visual acuity in
children ?

 Most eye problems can be treated if
detected early .

 Useful in decision making .

 To know if visual development is normal .

 Helps decide eligibility for low vision and
rehabilitation services .

Types :

 There are at least two types of visual acuity :-
 recognition acuity and
 resolution acuity.

 Recognition acuity relates to the detail in the smallest
letter, number or other shape that can be recognised.

 resolution acuity is the smallest separation between
dots or between bars in a grating that can be resolved.

TECHNIQUES FOR VISUAL
ACUITY QUANTITATION

1. Tests for indirect assessment of vision :

a) Historical and observational tests,
b) Binocular fixation preference and fixation targets,
c) CSM method.

2. Tests for recognition acuity :

a) Dot visual acuity,
b) Coin test
c) Miniature toy test
d) Marble game test
e) Sheridans ball test
f) Bock’s candy test (100’s and 1000’s test)
g) Worth ivory ball test
h) others

3) Tests for resolution acuity :

a) Opticokinetic nystagmus,
b) Preferential looking test,
c) Cardiff acuity cards,
d) Visual evoked potentials

 Children in this age group generally perform best if the
examination takes place when they are alert.

 Examination early in the morning or after an infant's
nap is usually most effective.

 Because infants tend to be more cooperative and alert
when feeding, it is also helpful to suggest that the parent
bring a bottle for the child.

Tests for indirect assessment of vision

- Historical and observational tests,
- Binocular fixation preference and fixation targets,
- CSM method.

HISTORICAL AND OBSERVATIONAL TECHNIQUES :

 Parents or caretakers are asked routinely whether
the child responds to a silent smile, enjoys silent
mobiles, and follows objects around the
environment.

 Pertinent observations include strabismus, nystagmus,
persistent staring, and inattention to objects

 For example, when a unilateral, constant strabismus is
present, visual acuity is presumed to be reduced in the
strabismic eye.

 In the presence of a constant, alternating strabismus, visual
acuity is likely to be normal in both eyes.

 The pupillary light response is not equivalent to visual
ability, but its presence indicates intact afferent visual
neurologic pathways to the level of the brachium of the
superior colliculus and efferent pathways to the iris
sphincter.

 This reflex is present in premature babies over 29–31weeks
of gestational age.

 Visualization in very young children sometimes requires a
magnifying glass, as their pupils are smaller than those of
older children (because of decreased sympathetic tone) and
the light responses are of small amplitude.

 The blink to a bright light is a behavior learned by
30weeks of gestational age and occasionally is present
in decorticate infants. The blink to a threatening
gesture is another learned reflex, usually present by
5months.

 when testing, care must be taken not to brush air against the
child’s corneas and elicit a blink by that mechanism.

 Another behavior that is unique to babies is “eye
popping.” Sometimes, for a variety of reasons, very
young infants don't show any distinguishable visual
behavior at all. In this case, the eye popping reflex
indicates at least the infant’s ability to detect changes
in room illumination.

 When the room lights are suddenly dimmed, the baby's
upper eye lids should pop open wide for a moment.
The baby will often close its eyes when the lights are
brought back up, but will again pop its eyes open when
the lights are dimmed. This behavior is documented as
"positive eye popping".

FIXATION TARGETS (fix and follow) :
 If appropriate targets are used, this reflex can be demonstrated
by about 6 wk of age.

 The test is performed by seating the child comfortably in the
caretaker's lap. The object of visual interest, usually a bright-
colored toy, is slowly moved to the right and to the left. The
examiner observes whether the infant's eyes turn toward the
object and follow its movements (fix and follow behavior) . The
examiner can use a thumb to occlude one of the infant's eyes in
order to test each eye separately.

 If the child has a f/f behaviour then it is assumed that the patient
could see a small target or toy in a normally illuminated room.

 The human face is a better target than test objects. If
the appropriate following movements are not
elicited, the test should be repeated with the
caretaker's face as the test stimulus.

 It should be remembered that even children with poor
vision may follow a large object without apparent
difficulty, especially if only one eye is affected.

Binocular fixation preference :

Behavioral evidence of decreased vision in right eye. (A) A small toy is used to get the child’s
attention, and the examiner covers the right eye to monitor fi xation of the left eye. The child fi
xates on the toy without objecting. (B) When the left eye is covered, the child objects and tries
to move the examiner’s hand. (C) When the right eye is covered, the child does not object and
tracks the object.

Some children object to having either eye covered, simply because they
do not like having the examiner’s hand near their face. If this is the
case, this test cannot accurately determine whether there is a difference
in vision between the eyes.

CSM METHOD :
 It is done with one eye fixating on an accommodative target held at
40cm

 ‘C’ refers to the location of corneal light reflex as the patient fixates the
examiner’s light under monocular conditions. Normally reflected light
from cornea in near the centre of the cornea and it should be positioned
symmetrically in both eyes. If fixation target is viewed eccentrically,
fixation is termed uncentral.

 ‘S’ refers to steadiness of fixation on examiners light as it is held
motionless and also as it is slowly moved about.

 ‘M’ refers to the ability of the patient to maintain alignment first with one
eye, then with the other, as the opposite eye in uncovered. Maintenance
of fixation is evaluation under binocular conditions. Inability to maintain
fixation with either eye, with opposite eye uncovered is presumptive
evidence of a difference in acuity between the two eyes.

 Evaluation :

 CSM – 6/9 – 6/6
 CSNM –6/36 – 6/60
 Unsteady central fixation < 6/60

Tests for recognition acuity
Dot visual acuity Kay pictures
Coin test LEA symbols
Miniature toy test Ffooks symbols
Marble game test Sheridan gardner single letter
Worth ivory ball test optotypes
Bock’s candy test Sonksen Silver acuity system

 Dot visual acuity test : child is shown an illuminated
box with black dots of different sizes printed on it. The
smallest dot identified denotes the visual acuity of the child.

 Coin test : Child is asked to identify two faces of coins of
different size held at different distance.

 Miniature toy test :Child is shown a miniature toy from
a distance of 10 feet and asked to name / pick the pair from
assortment.

 Marble game test : The child is asked to place marbles
in holes of a card or in a box. It compares the functioning of
the child’s eye when one or the other is closed and vision is
noted as useful or less useful.

 Worth Ivory ball tests : Ivory balls 0.5 to 2.5" in
diameter are rolled on the floor in front of the child and he is
asked to retrieve each. Acuity is estimated on the basis of
smallest size for the test distance.

 Bock’s candy bead test : Snellen equivalent of
6/60 is estimated by this method. The child is asked to match
pick up beads 1mm size at 40 cm.

Examples of recognition acuity. A. Kay
pictures B. LEA symbols.

Methods of visual acuity testing in preverbal children

Tests for resolution acuity
Opticokinetic nystagmus
Preferential looking test
Cardiff acuity cards
Visual evoked potentials

OPTICOKINETIC NYSTAGMUS :

 Evaluation of the presence or absence of
opticokinetic nystagmus was the first “technologic”
approach to acuity measurement in preverbal
children.

 Acuity was measured binocularly while the infant was positioned on his or
her back in a crib looking up at a canopy of black and white stripes. During
testing, the stripes moved in an arc across 180 O of the infant’s visual field.

 Patient follows the stripe
with a Slow motion and As it
disappears,Suddenly picks up
A new stripe.

 An assessment of visual
Acuity is made by varying the
Width of stripes or the distance
From the drum.

Advantages :

 As the testing drums are reasonably priced, portable,
and rarely break, this technique remains in use as a
quick and easy method with which to evaluate infant
acuity.

Disadvantages / limitations :
 disturbing fact is the realization that normal responses[6] may
occur in the occasional decorticate infant, which indicate that
subcortical areas of the occipital cortex may generate
opticokinetic responses.

 If one uses the readily available handheld optokinetic nystagmus
drum or tape, it is difficult to keep the infant fixated on this
stimulus which takes up only a small portion of his or her visual
environment.

 smudges, distortions, unequal stripe widths or any imperfection
in the stimulus that the eye can resolve may elicit false optokinetic
nystagmus responses in a testing circumstance.

 Moreover, it is essential that the stimuli have uniform space-
average luminance and be moved at a uniform rate across the
visual field. Obviously, most testing devices available in
ophthalmology clinics and examining rooms do not meet these
rigid requirements.

 eye movements evoked by dot stimuli can be suppressed
and poor correlation has been found between the acuity
measurements obtained by this technique and those
obtained by a standard Snellen assessment.

 Finally, it is important to note that the use of optokinetic
nystagmus to assess visual acuity may lead to errors of
interpretation, because one is evaluating a motor response
in an attempt to assess sensory function. The absence of
optokinetic nystagmus may be due to some alteration in the
ocular motor systems necessary to generate this eye
movement and not to the patient’s failure to “see” the
stimuli

FORCED CHOICE PREFERENTIAL LOOKING :

 The FPL technique was conceived by David Teller.

 This testing technique is based on the observation that infants
demonstrate a greater tendency to fix a pattern stimulus than a
homogeneous field.

 They measure resolution acuity, using either a grating target as
with the Teller cards or the vanishing optotype principle, as
with the more recently Developed Cardiff Acuity Cards.

 CAC were used from a distance of 50cm and LG were evaluated from 57cm
with a luminance of 23 cd/m2 as recommended by the manufacturers.

 Preferential looking involves showing the infant two stimuli, a grating
composed of black and white stripes (or other quantitated patterns), and a
grey screen of equal space-average luminance.

 An observer, unaware of the location of the patterned stimuli, is positioned
behind a peephole located centrally between the grating and the
homogeneous field.

 The observer monitors the direction of the child’s eyes and head during
stimulus presentation. The position and width of the stripes are varied on
each trial.

 Acuity is estimated by determining the smallest striped width to which the
infant will show differential fixation of the grating as opposed to the
homogeneous field i.e The frequency of the line spacing determines the
visual acuity.

 The threshold is usually defined as when the observer
is correct 75% of the time.

 This technique becomes a “forced choice” method
when the observer has to decide, based on their
observation of the child’s head and eye movements,
where the stimulus is located.

 The Cardiff Test is good for slightly older children (18
- 60 months). It consists of different cards, which are
held in front of the child. Each has a picture in the
upper or the lower part of the card. If the child looks
towards the picture on the card, you note the size as
detected.

 In Cardiff Acuity Card , the targets are pictures drawn with a white band
bordered by two black bands, all on a neutral grey background. The
average brightness of the picture is equal to that of the grey background. If
the child’s vision is good enough to resolve the white and black bands, the
picture will be visible but if the bands are too narrow for the child to
resolve them, the picture merges with the grey background, and simply
becomes invisible. (vanishing optotypes)

Advantages :

 Testing cards are simple, portable, and cannot lose
calibration; in a typical child, the testing of both
eyes often takes less than 20min.

Disadvantages / limitations :

 The child must be alert and able to generate neck and eye
movements, which disqualifies many whose hypotonia and
inattention prevent such purposeful movement – a significant
limitation in the evaluation of developmentally delayed infants.

 Thus, as with the opticokinetic nystagmus technique, vision is
evaluated by means of a motor response.

 In addition, this test presents a resolution acuity task, not a
recognition acuity task, and thus may be less ideal for the
detection of amblyopia than the visual evoked response test.
While Tellers card measures only grating acuity which is better in
than resolution acuity in children.

 As the cards can be presented with the stripes in one
orientation (vertical) only, the acuities of some optically
uncorrected astigmatic children may be estimated
erroneously using this technique.

 Children who have nystagmus may be unable to fixate on
the targets accurately, and those who have visual field
defects may have difficulty finding the targets.

 False high acuities are detected in patients with
anisometropia and strabismic amblyopia as these patients
typically have better near visual acuity.

 Lack of crowding phenomenon.

VISUAL EVOKED POTENTIALS :
 Visual evoked potentials (VEPs) are electrical brain responses that
are triggered by the presentation of a visual stimulus. VEPs are
distinguished from the spontaneous electroencephalogram (EEG)
due to their consistent time of occurrence after the presentation
of the stimulus (time-locking).

 The surface-recorded VEP reflects the activity of cortical visual
areas, with contributions from subcortical generators being
apparent only under highly specialized recording conditions

 evoked potentials (EPs) are so small compared to the background
EEG activity that the responses to a number of stimuli have to be
recorded and averaged with a computer in order to permit their
recognition and definition. The background EEG activity, which
has no fixed temporal relationship to the stimulus, is averaged out
by this procedure.

 Types :

1. Flash VEPs
2. Pattern reversal VEPs
3. Sweep VEPs

Procedure :

 A proprietary disposable headband with integrated electrodes is
used for recordings.
 The headband aligned the occiput (Oz), the mid-forehead
(Fpz), and the temple (ground). Skin contact with the pre-gelled
electrodes is enhanced with a small amount of EEG conductive
paste.
 Infants are positioned on a parent’s lap and children are seated in
a comfortable chair at a measured distance of 57 cm from a 17-
inch (43-cm) display monitor, so that the stimulus subtended a
total visual angle of 20o.
 The room is darkened except for the light from the testing
equipment.
 Testing is performed monocularly, using an adhesive occluder
over the fellow eye.

Test stimulus, showing a cartoon
figure (top), which appears
before the sequence of gratings
(sample at bottom).

Flash VEP :
 It is possible to record a VEP to simple flashes of light. This is useful where
the visual acuity is too poor to perceive even large checks or where fixation
or concentration is poor.

 The flash VEP is much less macula
-dominated than the pattern VEP and
can be recorded through cataracts or
corneal scars. Unfortunately, it is much
more variable in appearance than the
pattern VEP and, in general, it is used
as a basic indication of the integrity of
The visual pathway from the eye to the
occipital cortex.

 The most consistent feature is a positive component, designated P2, which
usually occurs at around 125 ms after the flash, but identification of even
this component is not always easy

PATTERN VEP :

 Pattern VEP can be used to measure VA by measuring the
response amplitude of the VEP at each frequency, for a
range of spatial frequencies.

 The amplitude is plotted against spatial frequency and a
regression fit is used to determine the point at which the
response would become zero, which is used as an estimate
of VA.

 Pattern reversal VEP is measured using checker board.

 In it the pattern of stimulus is changed ( black squares go
white and white squares go black) with the overall
illumination remaining same.

 It depends on form sense and thus gives a rough estimate of
visual acuity.

Evaluation :

 When the size of the checks is reduced to the point
where the contrast borders can no longer be resolved,
the cortical response disappears.

 At a check size which subtends a visual angle of 15 min
arc, a visual acuity of approximately 6/18–6/24 is
required for a clear cortical response.

 At a check size of 60 min arc, a visual acuity of 3/60–
6/60 is required for a clear cortical response.

 The component of major clinical importance is the so-called
P100 response, a positive peak having a latency of
approximately 100 ms. Its presence, latency, and symmetry
over the two sides of the scalp are noted. Amplitude may
also be measured, but changes in size are much less helpful
for the recognition of pathology. VEPs are most useful in
detecting dysfunction of the visual pathways anterior to the
optic chiasma.

Sweep VEP :
 Sweep VEP essentially performs the same operation, but the
spatial frequencies are varied very quickly over time and
the amplitudes are immediately plotted with respect to
spatial frequency (or time). For example, to measure VA, the
spatial frequency changes from low to high in about 10-20
seconds. The regression line of the response amplitude is
extrapolated to zero, which gives a measure of the VA.

METHODS OF VEP RECORDING :
 Time-locked responses to abrupt presentations are referred to as
transient VEPs.

 A second method of recording VEPs, the steady-state method, uses
temporally periodic stimuli. For commonly used pattern reversal
stimuli, the frequency of the repetition is often specified as the
pattern reversal rate in reversals per sec. This rate is twice the
stimulus fundamental frequency (in Hz), which is more commonly
used to describe the temporal frequency of pattern onset–offset
stimuli.

 As the stimulus repetition rate increases, the responses to
successive stimuli begin to overlap. At high stimulation rates, the
response is comprised of only a small number of components that
occur at exact integer multiples of the stimulus frequency. Activity
at each of the frequency components of the steady-state response is
characterized by its amplitude and phase, where phase represents
the temporal delay between the stimulus and the evoked response.

 It is important to recognise that the exact measure of VA
obtained with sVEP or ssVEP depends on many parameters;

 whether data from electrodes is averaged or the best value
is taken,
 whether a checkerboard or a grating is used, and
 the exact method for estimating the acuity threshold.

LIMITATIONS :
 cumbersome process of attaching and standardizing
electrodes,

 the time required for testing,

 Expensive instruments,

 the relatively monotonous stimuli,

 the complexity of the generated waveforms,hence requires
training ,

 VEP are recorded even in absence of occipital cortex and in
cortical blindness due to contribution by sec. visual cortices.

Selecting the appropriate clinical test :


 Because a child can vary significantly from expected age norms, it is
important not to rely solely upon chronological age when choosing
testing procedures. Appropriate test procedures need to be based on the
child's developmental age and specific capability.

Methods of visual acuity testing in preverbal children

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